Radio communication method, user terminal, radio base station and radio communication system

ABSTRACT

The present invention is designed so that it is possible to feed back channel state information (CSI) that is suitable for downlink communication in which 3D beams are used. The radio communication method of the present invention is a radio communication method to allow a radio base station to carry out downlink communication with a user terminal by using a 3D beam that is formed with a horizontal beam having directivity in a horizontal plane and a vertical beam having directivity in a vertical plane, and has the steps in which the radio base station transmits a plurality of measurement reference signals that are pre-coded using different precoding weights between a plurality of vertical beams, and the user terminal transmits channel state information that is generated based on the plurality of measurement reference signals, to the radio base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of and, thereby,claims benefit under 35 U.S.C. §120 to U.S. patent application Ser. No.14/426,321 filed on Mar. 5, 2015, titled, “RADIO COMMUNICATION METHOD,USER TERMINAL, RADIO BASE STATION AND RADIO COMMUNICATION SYSTEM,” whichis a national stage application of PCT Application No.PCT/JP2013/070784, filed on Jul. 31, 2013, which claims priority toJapanese Patent Application No. 2012-197745 filed on Sep. 7, 2012. Thecontents of the priority applications are incorporated by reference intheir entirety.

TECHNICAL FIELD

Embodiments of the invention relate to a user terminal in anext-generation mobile communication system.

BACKGROUND ART

In a UMTS (Universal Mobile Telecommunications System) network,long-term evolution (LTE) is under study for the purposes of furtherincreasing high-speed data rates, providing low delay, and so on(non-patent literature 1). In LTE, as multiple access schemes, a schemethat is based on OFDMA (Orthogonal Frequency Division Multiple Access)SC-FDMA (Single Carrier Frequency Division Multiple Access) is used onin uplink channels (uplink).

In LTE, MIMO (Multi Input Multi Output), which achieves improved datarates (spectral efficiency) by transmitting and receiving data using aplurality of antennas, is defined. In MIMO, a plurality oftransmitting/receiving antennas are provided in thetransmitter/receiver, so that different information sequences aretransmitted from different transmitting antennas at the same time.Meanwhile, on the receiving side, taking advantage of the fact thatfading variation is produced differently between thetransmitting/receiving antennas, information sequences that have beentransmitted at the same time are separated and detected.

As MIMO transmission schemes, single-user MIMO (SU-MIMO), in whichtransmission information sequences for the same user are transmitted atthe same time from different transmitting antennas, and multi-user MIMO(MU-MIMO), in which transmission information sequences for differentusers are transmitted at the same time from different transmittingantennas, have been proposed. In SU-MIMO and MU-MIMO, optimal PMIs(Precoding Matrix Indicators) to match the amount of phase and amplitudecontrol (precoding weights) to be set in the antennas are selected fromcodebooks, and fed back to the transmitter as channel state information(CSI). On the transmitter side, each transmitting antenna is controlledbased on the CSI fed back from the receiver, and transmissioninformation sequences are transmitted.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: 3GPP TR 25.913 “Requirements for Evolved    UTRA and Evolved UTRAN”

SUMMARY OF INVENTION

Also, successor systems of LTE (referred to as, for example,“LTE-advanced” or “LTE enhancement” (hereinafter referred to as“LTE-A”)) are under study for the purpose of achieving furtherbroadbandization and increased speed beyond LTE. In this LTE-A, a studyis also in progress to carry out downlink communication (for example,MIMO transmission) by using 3D beams that have directivity in thevertical plane in addition to the horizontal plane. Consequently,realization of a feedback scheme for channel state information (CSI)that is suitable for downlink communication using 3D beams is awaited.

The present invention has been made in view of the above, and it istherefore an object of the present invention to provide a radiocommunication method, a user terminal, a radio base station and a radiocommunication system that can feed back channel state information (CSI)that is suitable for downlink communication using 3D beams.

The radio communication method of the present invention is a radiocommunication method to allow a radio base station to carry out downlinkcommunication with a user terminal by using a 3D beam that is formedwith a horizontal beam having directivity in a horizontal plane and avertical beam having directivity in a vertical plane, and this radiocommunication method includes the steps in which the radio base stationtransmits a plurality of measurement reference signals that arepre-coded using different precoding weights between a plurality ofvertical beams, and the user terminal transmits channel stateinformation that is generated based on the plurality of measurementreference signals, to the radio base station.

According to the present invention, it is possible to feed back channelstate information (CSI) that is suitable for downlink communicationusing 3D beams.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram to show examples of mapping of CRSs;

FIG. 2 is a diagram to show examples of mapping of CSI-RSs;

FIG. 3 is a diagram to explain a communication scheme (3D MIMO/beamforming) that may be applied to an LTE-A system;

FIGS. 4A and 4B provide diagrams to show schematic views of a 2D antennaand a 3D transmitting antenna;

FIGS. 5A and 5B provide diagrams to explain PMIs in 2D channels and 3Dchannels;

FIGS. 6A and 6B provide diagrams to explain examples of 3D antennastructures;

FIGS. 7A, 7B, and 7C provide diagrams to explain a horizontal beam/avertical beam/a 3D beam;

FIG. 8 is a diagram to explain horizontal domain channels produced by aplurality of vertical beams;

FIG. 9 is a diagram to explain a radio communication method according toexample 1.1 of the present embodiment;

FIG. 10 is a diagram to explain a radio communication method accordingto example 1.2 of the present embodiment;

FIG. 11 is a diagram to explain a radio communication method accordingto example 1.3 of the present embodiment;

FIG. 12 is a diagram to explain a radio communication method accordingto example 2.1 of the present embodiment;

FIG. 13 is a diagram to explain radio communication method according toexample 2.2 of the present embodiment;

FIG. 14 is a diagram to explain a system structure of a radiocommunication system according to the present embodiment;

FIG. 15 is a diagram to explain an overall structure of a radio basestation according to the present embodiment;

FIG. 16 is a diagram to explain an overall structure of a user terminalaccording to the present embodiment;

FIG. 17 is a diagram to explain functional configurations of a radiobase station according to the present embodiment; and

FIG. 18 is a diagram to explain functional configurations of a userterminal according to the present embodiment.

DESCRIPTION OF EMBODIMENTS

Now, measurement reference signals to be used for channel stateinformation (CSI) feedback (hereinafter referred to as “CSI feedback”)in the LTE system and the LTE-A system will be described with referenceto FIG. 1 and FIG. 2. For the measurement reference signals, CRSs(Common Reference Signals) and CSI-RSs (CSI-Reference Signals) and so onare used.

CRSs are measurement reference signals that were introduced in Rel-8 forthe purposes of CSI feedback, cell search and so on. CRS signalsequences are pseudo-random sequences, and are subjected to QPSKmodulation. QPSK-modulated CRSs are mapped to a plurality of resourceelements (REs) in accordance with predetermined rules. Note that CRSsare not user-specific reference signals like DMRSs (DeModulationReference Signals), but are cell-specific reference signals, andtherefore are not pre-coded.

FIG. 1 is a diagram to show examples of mapping of CRSs when the numberof antenna ports is one, two and four. As shown in FIG. 1, CRSs ofmaximum four antenna ports (which are numbered 0 to 3) are supported, sothat channel estimation for maximum four channels can be carried out ina user terminal UE. The CRSs (R₀ to R₃) of each antenna port are mappedto mutually different resource elements (REs), and areorthogonally-multiplexed by time division multiplexing (TDM)/frequencydivision multiplexing (FDM).

Meanwhile, CSI-RSs are measurement reference signals that wereintroduced in Rel-10 for the purpose of CSI feedback. CSI-RS signalsequences are pseudo-random sequences and subjected to QPSK modulation.QPSK-modulated CSI-RSs are mapped to CSI-RS resources. Note that CSI-RSsare not pre-coded either.

FIG. 2 is a diagram to show examples of mapping of CSI-RSs. As shown inFIG. 2, CSI-RSs of maximum eight antenna ports (which are numbered 15 to22) are supported, so that channel estimation for maximum eight channelscan be carried out in a user terminal UE. The CSI-RSs (R₁₅ to R₂₂) ofeach antenna port are orthogonally-multiplexed by time divisionmultiplexing (TDM)/frequency division multiplexing (FDM)/code divisionmultiplexing (CDM).

For example, in FIG. 2, the CSI-RSs of the antenna ports 15 and 16 (R₁₅and R₁₆) are mapped to the same resource elements (REs) andcode-division-multiplexed (CDM). The same holds with the CSI-RSs of theantenna ports 17 and 18 (R₁₇ and R₁₈), the CSI-RSs of the antenna ports19 and 20 (R₁₉ and R₂₀) and the CSI-RSs of the antenna ports 21 and 22(R₂₁ and R₂₂).

Note that, although FIG. 2 illustrates a case where the number ofantenna ports is eight, the CSI-RSs when the number of antenna ports isone, two and four are supported as well. In such cases, a nest structureis employed, so that the CSI-RS of each antenna port are mapped to evena larger number of resource elements (REs).

Also, in Rel-11 CoMP, a plurality of CSI processes are provided inassociation with a plurality of CoMP cells, respectively. CSI-RSresources are provided and CSI feedback is sent, for each of these CSIprocesses. Also, for each CSI process, a CSI configuration, which showsthe arrangement pattern of CSI-RS resources (in FIG. 2, CSIconfiguration 0), is defined.

Now, 3D beam forming will be described with reference to FIG. 3. FIG. 3is a conceptual diagram of MIMO transmission in which 3D beam forming isused. As shown in FIG. 3, in MIMO transmission using 3D beam forming, 3Dbeams having directivity in the vertical plane in addition to thehorizontal plane are output from the transmitting antennas of the radiobase station eNB. By means of 3D beams having varying directivities inthe vertical direction, the cell C1 of the radio base station eNB issectorized into an inner cell C2 and an outer cell C3.

In FIG. 3, the radio base station eNB outputs 3D beams B1 and B2 to userterminals UE1 and UE2 located in the inner cell C2, respectively, andcarries out downlink MIMO transmission. Meanwhile, the radio basestation eNB outputs 3D beams B3 and B4 to user terminals UE3 and UE4located in the outer cell C3, respectively, and carries out downlinkMIMO transmission. These 3D beams are made possible by means of 3Dantennas.

In 3D beam forming, it might occur that, compared to 2D beam forming,the number of transmitting antenna elements (Tx antenna elements)provided in the radio base station eNB increases significantly. FIG. 4shows schematic views of a 2D antenna and a 3D antenna.

FIG. 4A shows a schematic view of a 2D antenna, and FIG. 4B shows aschematic view of a 3D antenna. As shown in FIG. 4A, a 2D antenna isformed with a plurality of antenna elements (4Tx) that are providedalong the horizontal direction. As shown in FIG. 4B, a 3D antenna isformed with a plurality of antenna elements (4Tx) that are providedalong the horizontal direction, and a plurality of antenna elements(4Tx) that are provided along the vertical direction.

That is, the number of antenna elements (n_(H)) constituting the 2Dantenna is four (four in one row), and the number of antenna elements(n_(H) n_(V)) constituting the 3D antenna is sixteen (four in one rowand four in one column).

Now, PMIs in 2D channels used in 2D beam forming and 3D channels used in3D beam forming will be described with reference to FIG. 5. FIG. 5Ashows PMIs in a 2D channel (conventional PMIs), and FIG. 5B shows thePMI_(H)'s of horizontal domains (horizontal planes) in a 3D channel whenthe present embodiment is employed.

In FIG. 5B, each vertical beam (for example, the k-th vertical beam(W_(v) ^((k)))) forms the horizontal domain channel (horizontal channel)corresponding to that vertical beam, and the PMI_(H) of that channel isshown. That is, a PMI_(H) is equivalent to a 2D channel PMI. In the caseillustrated in FIG. 5B, from the user terminal UE side, it is possibleto see the 3D channel as the state of a 2D horizontal channelcorresponding to a predetermined vertical beam, by precoding ofmeasurement reference signals.

Next, an example of a 3D antenna structure will be described withreference to FIG. 6. To realize 3D beam forming/3D MIMO, a plurality ofantenna elements need to be placed in both the horizontal domain(horizontal plane) and the vertical domain (vertical plane). FIG. 6shows examples of 3D antenna structures. FIG. 6A shows a case whereantenna elements that are designed on a reduced scale are placed alongthe horizontal domain and the vertical domain, and FIG. 6B shows across-polarized antenna, in which antenna elements are placed to crossone another in the horizontal domain and the vertical domain.

A horizontal beam can be formed by pre-coding a plurality of horizontalantenna elements placed in the horizontal domain (or a horizontalantenna element sequence (the antenna elements to constitute one row inFIG. 6)) using a horizontal precoder (W_(H)). For example, as shown inFIG. 7A, a horizontal beam is formed to have an angle θ_(H) in thehorizontal domain, based on a horizontal precoder (W_(H)).

A vertical beam can be formed by pre-coding a plurality of verticalantenna elements placed in the vertical domain (or a vertical antennaelement sequence (the antenna elements to constitute one column in FIG.6)) using a vertical precoder (W_(V)). For example, as shown in FIG. 7B,a vertical beam is formed to have an angle θ_(V) in the vertical domain,based on a vertical precoder (W_(V)). Also, a plurality of verticalantenna elements or a vertical antenna element sequence may beconfigured to have the same polarized elements in the vertical domain(or in the vertical direction).

A 3D beam can be formed using a plurality of antenna elements that areplaced along the horizontal direction and the horizontal direction, bycombining a horizontal precoder (W_(H)) and a vertical precoder (W_(V)).For example, as shown in FIG. 7C, a 3D beam can be formed by combiningthe horizontal beam of FIG. 7A and the vertical beam of FIG. 7B.

Given the situation where three vertical beams are formed, FIG. 8 showsthe horizontal domain channels (horizontal channels) produced by eachvertical beam. To be more specific, in FIG. 8, the channels in thehorizontal domain that are produced by the vertical beam 1, the verticalbeam 2 and the vertical beam 3, respectively, are shown. As notedearlier, the vertical beams 1 to 3 are formed by pre-coding verticalantenna elements (vertical antenna element sequence) using verticalprecoders (W_(v) ⁽¹⁾, W_(v) ⁽²⁾ and W_(v) ⁽³⁾), respectively.

In downlink communication such as described above in which 3D beams areused, a study is in progress to pre-code measurement reference signalsthat respectively correspond to a plurality of vertical beams, usingdifferent precoding weights between these plurality of vertical beams,and map these plurality of measurement reference signals to the sameradio resources (for example, to resource elements of the same antennaport), so that it is possible to prevent the overhead of measurementreference signals from increasing. Consequently, realization of a CSIfeedback scheme that is based on a plurality of pre-coded measurementreference signals is awaited.

So, the present inventors have studied a radio communication method thatmakes it possible to feed back CSI based on a plurality of measurementreference signals that are pre-coded using different precoding weightsbetween a plurality of vertical beams, in downlink communication inwhich 3D beams formed with horizontal beams and vertical beams are used,and arrived at the present invention.

With the radio communication method according to the present embodiment,the radio base station eNB transmits a plurality of measurementreference signals that are pre-coded using different precoding weightsbetween a plurality of vertical beams. Also, a user terminal UEtransmits channel state information (CSI) that is generated based on theabove plurality of measurement reference signals, to the radio basestation eNB. Based on this CSI, the radio base station eNB selects theprecoding weights to form the vertical beams that are used in downlinkcommunication with the user terminal UE.

Note that, although, with the radio communication method according tothe present embodiment, a plurality of measurement reference signals arepre-coded using different precoding weights between a plurality ofvertical beams and mapped to the same radio resources, this is by nomeans limiting. For example, it is equally possible to pre-code aplurality of measurement reference signals using different precodingweights between a plurality of horizontal beams and map the referencesignals to the same radio resources. That is, the relationship betweenvertical beams and horizontal beams may be switched wheneverappropriate.

Also, although the radio communication method according to the presentembodiment uses, for example, CSI-RSs and CRSs as measurement referencesignals, this is by no means limiting.

First Embodiment

With the first embodiment, a case will be described where CSI-RS areused as measurement reference signals. With the first embodiment, aplurality of CSI-RSs that are pre-coded with different precoding weightsper vertical beam are mapped to different CSI-RS resources per verticalbeam. Each CSI-RS resource is associated with one vertical beam. By thismeans, CSI that is fed back with respect to each CSI-RS resourcerepresents the CSI of vertical beams, which are associated with eachCSI-RS resource, in the horizontal domain. Also, each CSI-RS resource isassociated with one CSI process.

Here, CSI-RS resources refer to radio resources that are determined inadvance for the mapping of CSI-RSs. For example, as shown in FIG. 2,CSI-RS resources are formed with a predetermined number of resourceelements (REs) that are determined per antenna port. Also, CSI-RSresources have a plurality of arrangement patterns (CSI configurations).

Note that, in Rel-11 CoMP, each CSI-RS resource is associated with oneCoMP cell, and furthermore corresponds to one CSI process. In this way,by associating a CSI-RS resource that is associated with one CoMP cellwith one vertical beam, it is possible to re-use the CSI feedbackconfiguration in Rel-11 CoMP in the CSI feedback configuration in 3Dbeam forming.

Example 1.1

The radio communication method according to a first example of the firstembodiment (hereinafter referred to as “example 1.1”) will be describedwith reference to FIG. 9. Note that, in FIG. 9, K≧1) vertical beams areconfigured. Also, the K vertical beams correspond to K CSI processes,respectively.

As shown in FIG. 9, a radio base station eNB pre-codes CSI-RSs usingdifferent precoding weights per vertical beam, maps the pre-codedCSI-RSs to different CSI-RS resources per vertical beam, and transmitsthem (step S11). Note that the CSI-RS resources of each vertical beammay be placed in the same subframe n or may be placed in differentsubframes, as shown in FIG. 9.

Also, the radio base station eNB reports CSI process information to auser terminal UE (step S12). Here, the CSI process information isinformation related to the K CSI processes to which the K vertical beamscorrespond respectively, and is, for example, the number of CSI-RSprocesses and the corresponding to antenna ports. The CSI processinformation is reported by, for example, higher layer signaling such asRRC signaling.

The user terminal UE performs channel estimation of each vertical beam(CSI process) (step S13). To be more specific, in FIG. 9, the userterminal UE estimates the horizontal channels formed by each verticalbeam:

H ^((k)) (k=1, . . . , K)  [Formula 1]

A horizontal channel refers to a 2D channel in the horizontal domainthat is formed by a vertical beam.

The user terminal UE calculates CSI for each vertical beam (CSI process)based on the results of channel estimation (step S14). To be morespecific, the user terminal UE calculates the PMI_(H) ^((k))'s, RI_(H)^((k))'s and CQI_(H) ^((k))'s (k=1 . . . , K) of the K horizontalchannels formed respectively by the K vertical beams. Here, the PMI_(H)^((k)) is the precoding matrix indicator of the horizontal channelformed by the k-th vertical beam, and identifies the precoding weightused in the horizontal channel. Also, the RI_(H) ^((k)) is the rankindicator of the horizontal channel formed by the k-th vertical beam.Furthermore, the CQI_(H) ^((k)) is the channel quality indicator of thehorizontal channel formed by the k-th vertical beam.

The user terminal UE feeds back the CSIs of all vertical beams (CSIprocesses) to the radio base station eNB (step S15). To be morespecific, the user terminal UE feeds back the PMI_(H) ^((k))'s, RI_(H)^((k))'s and CQI_(H) ^((k))'s (k=1, . . . , K) of the K horizontalchannels formed respectively by the K vertical beams. Note that the CSIsmay be fed back in different subframes per vertical beam, or the CSIs ofa plurality of vertical beams may be fed back in the same subframe.

Based on the CSIs of all vertical beams (CSI processes) that are fedback, the radio base station eNB selects the vertical beams to use indownlink MIMO transmission, and carries out scheduling and precoding(step S16). To be more specific, the radio base station eNB selectsPMI_(v)'s using K CQI⁽¹⁾, . . . , CQI^((K)) using a predeterminedfunction (for example, argmax). Here, the PMI_(v) is a verticalprecoding matrix indicator for forming a vertical beam, and identifiesthe precoding weight that is used in a vertical precoder.

With the radio communication method according to example 1.1, K verticalbeams are associated with K CSI processes, respectively, and the CSIs ofall of the K vertical beams (CSI process) are fed back. In Rel-11 CoMP,a plurality of CSI processes that are associated with a plurality ofCoMP cells, respectively, are defined. Consequently, by associating aplurality of CSI processes with a plurality of vertical beams,respectively, instead of a plurality of CoMP cells, it is possible toreduce the load of system implementation, and realize a CSI feedbackconfiguration in 3D beam forming. Also, it is possible to securebackward compatibility with user terminals UE (legacy terminals) thatsupport up to Rel-11.

Example 1.2

The radio communication method according to a second example of thefirst embodiment (hereinafter referred to as “example 1.2”) will bedescribed with reference to FIG. 10. Note that, in FIG. 10, K (K≧1)vertical beams are configured. Also, the K vertical beams correspond toK CSI processes, respectively. Note that each CSI process is identifiedby a CSI process identifier (CSI process ID).

Steps S21, S23 and S24 in FIG. 10 are the same as steps S11, S13 and S14in FIG. 9, and therefore will not be described. In step S22 of FIG. 10,the radio base station eNB reports the number of vertical beams M (M≧1)that require CSI feedback, in addition to the CSI process informationdescribed with step S12 of FIG. 9, to the user terminal UE. Here, thenumber M is reported by, for example, higher layer signaling such as RRCsignaling. Note that the number M is configured in the user terminal UEin advance, the number M needs not be reported.

The user terminal UE selects the CSIs of the best M vertical beams fromthe CSIs of K vertical beams (CSI processes) calculated in step S24(step S25). To be more specific, the user terminal UE measures thereceived quality of each CSI-RS of the K vertical beams, and selects theCSIs of M vertical beams where good received quality is measured. Here,the RSRP (Reference Signal Received Power) is used as the receivedquality of the CSI-RSs, but it is equally possible to use the RSRQ(Reference Signal Received Quality), the SINR (Signal Interference plusNoise Ratio) and so on.

Note that, in step S25, the user terminal UE may select the CSIs of Mbest vertical beams, in which the horizontal channels formed by thevertical beams show good CQI_(H)'s. Also, the user terminal UE mayselect the CSIs of M vertical beams, in which the capacity in thesectors formed by the vertical beams is sufficient.

The user terminal UE feeds back the CSIs of M vertical beams (CSIprocesses) that are selected, and the CSI process IDs that identify theM CSI processes to the radio base station eNB (step S26). To be morespecific, the user terminal UE feeds back the PMI_(H) ^((k))'s, RI_(H)^((k))'s and CQI_(H) ^((k))'s (kεS_(M)) of M horizontal channels thatare formed by the M selected vertical beams respectively.

Based on the CSIs of the M vertical beams (CSI processes) that are fedback, the radio base station eNB selects the vertical beams to use indownlink MIMO transmission, and carries out scheduling and precoding(step S27).

With the radio communication method according to example 1.2, the CSIsof M vertical beams selected from the CSIs of K vertical beams (CSIprocesses) are fed back. Consequently, compared to the case where theCSIs of all of the K vertical beams are fed back (example 1.1), it ispossible to reduce the overhead due to CSI feedback.

Note that, with the radio communication method according to example 1.2,it is possible to further reduce the overhead due to CSI feedback byutilizing the characteristics of 3D beams. To be more specific, in stepS26, the user terminal UE may perform joint selection with respect tothe PMI_(H)'s/RI_(H)'s of horizontal channels formed respectively byneighboring vertical beams (CSI processes). Here, joint selection meansselecting the PMI_(H)/RI_(H) of a horizontal channel that is optimal formultiple neighboring vertical beams. In this case, it is possible toreduce the overhead compared to the case of feeding back MPMI_(H)'s/RI_(H)'s.

Also, in step S26, the user terminal UE may select the differentialvalues of the CQI_(H)'s of horizontal channels formed by neighboringvertical beams (CSI processes). In this case, once one CQI_(H) (forexample, the CQI_(H) of the best vertical beam) is fed back, only thedifferential values need to be fed back with respect to the other M−1CQI_(H)'s. Consequently, it is possible to reduce the overhead comparedto the case of feeding back M CQI_(H)'s.

Also, in step S26, the user terminal UE may feed back the CSI of thebest vertical beam (CSI process), the CSI of at least one vertical beamthat neighbors the best vertical beam and the CSI process IDcorresponding to that best vertical beam. In this case, only one CSIprocess ID is fed back, so that it is possible to reduce the overheadcompared to the case of feeding back M CSI process IDs.

Example 1.3

The radio communication method according to a third example of the firstembodiment (hereinafter referred to as “example 1.3”) will be describedwith reference to FIG. 11. Note that, in FIG. 11, K (K≧1) vertical beamsare configured. Also, the K vertical beams correspond to K CSI processesrespectively. Note that each CSI process is identified by a CSI processID.

Steps S31, S33 and S34 of FIG. 11 are the same as steps S11, S13 and S14of FIG. 9, and therefore will not be described. In step S32 of FIG. 11,the radio base station eNB reports a predetermined threshold value forCSIs that require CSI feedback, to the user terminal UE, in addition tothe CSI process information described with reference to step S12 of FIG.9. The predetermined threshold value may be reported by, for example,higher layer signaling such as RRC signaling. Also, when thepredetermined threshold value is configured in the user terminal UE inadvance, the predetermined threshold value needs not be reported.

The user terminal UE selects the CSIs of vertical beams having betterCSIs than a predetermined threshold value (step S35), from the CSIs ofthe K vertical beams (CSI processes) calculated in step S34. To be morespecific, the user terminal UE measure the received quality of eachCSI-RS of the K vertical beams, and selects the CSIs of vertical beamswhere better received quality than a predetermined threshold value ismeasured. As has been described above, the RSRP is used as the receivedquality of the CSI-RSs, but it is equally possible to use the RSRQ, theSINR and so on.

Note that, in step S35, the user terminal UE may select the CSIs of Mvertical beams, in which the horizontal channels formed by the verticalbeams show better CQI_(H)'s than a predetermined threshold value. Also,the user terminal UE may select the CSIs of vertical beams, in which thecapacity in the sectors formed by the vertical beams fulfills apredetermined threshold value.

The user terminal UE may feed back the CSIs of the selected verticalbeams (CSI processes), and the CSI process IDs that identify theselected CSI processes, to the radio base station eNB (step S36). To bemore specific, the user terminal UE feeds back the PMI_(H) ^((k))'s,RI_(H) ^((k))'s and CQI_(H) ^((k))'s (kεS) of the horizontal channelsformed by the selected vertical beams.

Based on the CSIs of the M vertical beams (CSI processes) that are fedback, the radio base station eNB selects the vertical beams to use indownlink MIMO transmission, and carries out scheduling and precoding(step S37).

With the radio communication method according to example 1.3, among theCSIs of K vertical beams (CSI processes), only the CSIs of verticalbeams that are better than a predetermined threshold value are fed back.Consequently, it is possible to reduce the overhead due to CSI feedbackcompared to the case of feeding back the CSIs of all of the K verticalbeams (example 1.1).

Second Embodiment

Cases will be described with a second embodiment where CRSs are used asmeasurement reference signals. With the second embodiment, a pluralityof CRSs that are pre-coded with different precoding weights per verticalbeam are transmitted in different subframes per vertical beam. Eachsubframe is associated with one vertical beam. By this means, CSI thatis fed back with respect to each CSI-RS resource represents the CSI ofvertical beams, which are associated with each CSI-RS resource, in thehorizontal domain.

Example 2.1

The radio communication method according to a first example of thesecond embodiment (hereinafter referred to as “example 2.1”) will bedescribed with reference to FIG. 12. Note that, in FIG. 12, K verticalbeams are configured.

As shown in FIG. 12, the radio base station eNB pre-codes CRSs usingdifferent precoding weights per vertical beam, and transmits thepre-coded CRSs in different subframes per vertical beam (step S41). Forexample, in FIG. 12, the CRS that is pre-coded with the precoding weightcorresponding to the first vertical beam is transmitted in subframe n(step S41 ₁). Also, the CRS that is pre-coded with the precoding weightcorresponding to the k-th vertical beam is transmitted in subframe n+q₁(step S41 _(K)).

The user terminal UE performs channel estimation of each vertical beam,and calculate the CSI of each vertical beam based on the results ofchannel estimation (step S42). To be more specific, in FIG. 12, the userterminal UE estimates the horizontal channel formed by the firstvertical beam

H ⁽¹⁾  [Formula 2]

based on the CRS that is pre-coded with the precoding weightcorresponding to the first vertical beam, and calculates the PMI_(H)⁽¹⁾, RI_(H) ⁽¹⁾, and CQI_(H) ⁽¹⁾ of that horizontal channel (step S42₁). Similarly, based on the CRS that is pre-coded with the precodingweight corresponding to the k-th vertical beam, the horizontal channelformed by the K-th vertical beam

H ^((K))  [Formula 3]

is estimated, and the PMI_(H) ^((K)), RI_(H) ^((K)), and CQI_(H) ^((K))of that horizontal channel are calculated (step S42 _(K)).

The user terminal UE feeds back the CSIs of all vertical beams to theradio base station eNB (step S43). To be more specific, the userterminal UE feeds back the PMI_(H) ^((k))'s, RI_(H) ^((k))'s and CQI_(H)^((k))'s (kε1, . . . , K), calculated in steps S42 ₁ S42 _(K),respectively, in continuous subframes, separately (step S43 ₁ to S43_(K)).

Based on the CSIs of all vertical beams that are fed back, the radiobase station eNB selects the vertical beams to use in downlink MIMOtransmission, and carries out scheduling and precoding (step S44). To bemore specific, the radio base station eNB selects PMI_(v)'s using KCQI⁽¹⁾, . . . , CQI^((K)) using a predetermined function (for example,argmax).

With the radio communication method according to example 2.1, K verticalbeams are associated respectively with the CRSs of K subframes, and theCSIs of all of the K vertical beams are fed back. In this way, by usinga CSI feedback configuration based on CRSs, it is possible to reduce theload of system implementation and realize a CSI feedback configurationin 3D beam forming. Also, it is possible to secure backwardcompatibility with user terminals UE (legacy terminals) that support CSIfeedback based on CRSs yet do not support CSI feedback based on CSI-RSs.

Example 2.2

The radio communication method according to a second example of thesecond embodiment (hereinafter referred to as “example 2.2”) will bedescribed with reference to FIG. 13. Note that, in FIG. 13, K verticalbeams are configured.

Steps S51 ₁ to S51 _(K) in FIG. 13 are the same as steps S41 ₁ to S41_(K) in FIG. 12, and therefore will not be described. Note that,although not illustrated, the radio base station eNB reports apredetermined threshold value for CSIs that require CSI feedback, to theuser terminal UE. The predetermined threshold value may be reported by,for example, higher layer signaling such as RRC signaling. Also, whenthe predetermined threshold value is configured in the user terminal UEin advance, the predetermined threshold value needs not be reported.

The user terminal UE performs channel estimation of each vertical beam,calculate the CSI of each vertical beam based on the results of channelestimation, and determines whether the calculated CSIs are better than apredetermined threshold value (step S52). For example, in FIG. 13, thePMI_(H) ⁽¹⁾, RI_(H) ⁽¹⁾ and CQI_(H) ⁽¹⁾ of the horizontal channel formedby the first vertical beam are calculated, and whether or not thiscalculated CQI_(H) ⁽¹⁾ is better than a predetermined threshold value isdetermined (step S52 ₁). Here, the CQI_(H) ⁽¹⁾ that is calculated ispoorer than the predetermined threshold value, so that the CSI of thefirst vertical beam is not fed back.

Similarly, in FIG. 13, the PMI_(H) ^((K)), RI_(H) ^((K)) and CQI_(H)^((K)) of the horizontal channel formed by the K-th vertical beam arecalculated, whether or not the CQI_(H) ^((K)) that is calculated isbetter than a predetermined threshold value is determined (step S52 ₁).Here, the CQI_(H) ^((K)) that is calculated is better than thepredetermined threshold value, so that the CSI of the K-th vertical beamis fed back. To be more specific, the user terminal UE feeds back thePMI_(H) ^((K)), RI_(H) ^((K)), and CQI_(H) ^((K)) that are calculated,to the radio base station eNB (step S53).

Based on the CSIs of the vertical beams that are fed back, the radiobase station eNB selects the vertical beams to use in downlink MIMOtransmission, and carries out scheduling and precoding (step S54).

With the radio communication method according to example 2.2, only theCSIs of vertical beams that are better than a predetermined thresholdvalue, among the CSIs calculated based on the CRSs of K vertical beams,are fed back. Consequently, it is possible to reduce the overhead due toCSI feedback, compared to the case of feeding back the CSIs of all ofthe K vertical beams (example 2.1).

(Configuration of Radio Communication System)

Now, a radio communication system according to the present embodimentwill be described in detail.

FIG. 14 is a diagram to explain a system configuration of a radiocommunication system according to the present embodiment. Note that theradio communication system 1 shown in FIG. 14 is a system toaccommodate, for example, the LTE system or SUPER 3G. This radiocommunication system may execute carrier aggregation, whereby aplurality of component carriers, which are the system band of the LTEsystem, are aggregated. Also, this radio communication system may bereferred to as LTE-advanced (LTE-A), IMT-advanced, 4G and so on.

As shown in FIG. 14, the radio communication system 1 is configured toinclude a radio base station 10 and user terminals 20A and 20B thatcommunicate with the radio base station 10. The radio base station 10 isconnected with a higher station apparatus 30, and this higher stationapparatus 30 is connected with a core network 40. The higher stationapparatus 30 may be, for example, a gateway (GW), a mobility managemententity (MME) and so on, but is by no means limited to this.

As shown in FIG. 14, the radio base station 10 outputs 3D beams B1 andB2 that are formed by combining a horizontal beam having horizontaldirectivity and a vertical beam having vertical directivity. In FIG. 14,a plurality of 3D beams B1 and B2, having varying directivities in thevertical direction, form a plurality of sectors (an inner cell C2 and anouter cell C1). To be more specific, the 3D beam B1 having a small tiltangle forms the outer cell C1 that is distant from the radio basestation 10. Meanwhile, the 3D beam B2 having a large tilt angle formsthe inner cell C2 that is near the radio base station 10. Note that thetilt angle is the angle of beams with respect to the horizontaldirection (for example, the ground).

In FIG. 14, the user terminal 20A that is located in the outer cell C1carries out downlink communication with the radio base station 10 usingthe 3D beam B1. Also, the user terminal 20B located in the inner cell C2carries out downlink communication with the radio base station 10 usingthe 3D beam B2. MIMO transmission is used in this downlinkcommunication. The user terminals 20A and 20B may be either LTEterminals or LTE-A terminals, as long as they are user equipment (UE),which covers both mobile terminals and fixed terminals. The userterminals 20A and 20B hereinafter will be referred to as “user terminal20,” unless specified otherwise.

Note that, for radio access schemes in the radio communication system 1shown in FIG. 14, OFDMA (Orthogonal Frequency Division Multiple Access)is adopted on the downlink, and SC-FDMA (Single-Carrier FrequencyDivision Multiple Access) is adopted on the uplink, but the uplink radioaccess scheme is by no means limited to this.

Downlink communication channels include a PDSCH (Physical DownlinkShared Channel), which is shared between the user terminals 20 as adownlink data channel, and downlink L1/L2 control channels (PDCCH,PCFICH and PHICH). Transmission data and higher control information aretransmitted by the PDSCH. Scheduling information (DL grants and ULgrants) for the PDSCH and the PUSCH, and so on are transmitted by thePDCCH (Physical Downlink Control CHannel). Note that an enhanced PDCCH(also referred to as “E-PDCCH,” “ePDCCH,” “UE-PDCCH” and so on) that isfrequency-division-multiplexed with the PDSCH may be provided to solvethe shortage of capacity with the PDCCH.

Uplink communication channels include a PUSCH (Physical Uplink SharedChannel), which is shared between the user terminals 20 as an uplinkdata channel, and a PUCCH (Physical Uplink Control Channel), which is anuplink control channel. By means of this PUSCH, transmission data andhigher control information are transmitted. Also, downlink channel stateinformation (CSI), delivery acknowledgment information (ACK/NACK/DTX)and so on are transmitted by the PUCCH. Note that the channel stateinformation (CSI) and delivery acknowledgment information (ACK/NACK/DTX)may be transmitted by the PUSCH as well.

Next, overall configurations of the radio base station and userterminals according to the present embodiment will be described withreference to FIGS. 15 and 16.

FIG. 15 is a diagram to show an overall configuration of the radio basestation according to the present embodiment. As shown in FIG. 15, theradio base station 10 has a transmitting/receiving antenna 11, anamplifying section 12, a transmitting/receiving section 13, a basebandsignal processing section 14, a call processing section 15, and atransmission path interface 16. Note that, as shown in FIG. 6, thetransmitting/receiving antenna 11 is formed with a 3D antenna in whichantenna elements are aligned in both the horizontal domain and thevertical domain. To be more specific, the transmitting/receiving antenna11 has horizontal antenna element sequences that are each formed with aplurality of antenna elements and serve as horizontal beam formingunits, and vertical antenna element sequences that are each formed witha plurality of antenna elements and serve as vertical beam formingunits.

Downlink data for the user terminal 20 is input from the higher stationapparatus 30, into the baseband signal processing section 14, via thetransmission path interface 16. In the baseband signal processingsection 14, signal transmission processes are executed with respect tothe downlink data, including HARQ retransmission control, scheduling,transport format selection, channel coding, precoding, mapping to radioresources, an inverse fast Fourier transform (IFFT), and so on.

Also, in the baseband signal processing section 14, signal transmissionprocesses are carried out also with respect to downlink control data(for example, DCI), including channel coding, mapping to radioresources, an IFFT and so on. Furthermore, for broadcast informationprovided by broadcast channels and reference signals (CRSs, CSI-RSs,DM-RSs and so on), too, signal transmission processes such as mapping toradio resources, an IFFT and so on are carried out.

The transmitting/receiving section 13 converts baseband signals, whichare pre-coded and output from the baseband signal processing section 14per antenna element (see FIG. 6) in the transmitting/receiving antenna11, into a radio frequency band. The amplifying section 122 amplifiesthe radio frequency signals subjected to frequency conversion, andoutput the results through the transmitting/receiving antennas 111.

Meanwhile, as for uplink data from the user terminal 20, each radiofrequency signal that is received in the transmitting/receiving antenna11 is amplified in the amplifying section 12, and converted into abaseband signal through frequency conversion in thetransmitting/receiving section 13, and input in the baseband signalprocessing section 14.

In the baseband signal processing section 14, the uplink data that isincluded in the input baseband signals is subjected to signal receivingprocesses such as a fast Fourier transform (FFT), an inverse discreteFourier transform (IDFT) and error correction decoding, and transferredto the higher station apparatus 30 via the transmission path interface16. The call processing section 15 performs call processing such assetting up and releasing communication channels, manages the state ofthe radio base station 10 and manages the radio resources.

FIG. 18 is a diagram to show an overall configuration of a user terminalaccording to the present embodiment. As shown in FIG. 16, the userterminal 20 has a plurality of transmitting/receiving antennas 21, aplurality of amplifying sections 22, transmitting/receiving sections 23,a baseband signal processing section 24, and an application section 25.

As for downlink signals from the radio base station 10, radio frequencysignals received in the transmitting/receiving antennas 21 are eachamplified in the amplifying sections 22, and converted into basebandsignals through frequency conversion in the transmitting/receivingsections 23. The baseband signals are subjected to signal receivingprocesses such as FFT and error correction decoding in the basebandsignal processing section 24. In the downlink signals, downlink userdata is transferred to the application section 25 and subjected toprocesses related to higher layers.

Meanwhile, uplink data for the radio base station 10 is input from theapplication section 25 into the baseband signal processing section 24.In the baseband signal processing section 24, signal transmissionprocesses such as HARQ retransmission control, channel coding,precoding, a DFT and an IFFT are carried out, and the results areforwarded to each transmitting/receiving sections 23. The basebandsignals output from the baseband signal processing section 24 areconverted into a radio frequency band in the transmitting/receivingsections 23. After that, the radio frequency signals that have beensubjected to frequency conversion are amplified in the amplifyingsections 22 and transmitted from the transmitting/receiving antennas 21.

Next, the configurations of the radio base station and user terminalaccording to the present embodiment will be described in detail withreference to FIGS. 17 and 18. Note that, although FIG. 17 and FIG. 18show functional configurations pertaining to CSI feedback, otherfunctional configurations may be provided as well. Also, the functionalconfigurations shown in FIG. 17 are primarily provided in the basebandsignal processing section 14 of FIG. 15. Similarly, the functionalconfigurations shown in FIG. 18 are primarily provided in the basebandsignal processing section 24 of FIG. 16.

FIG. 17 is a diagram to show the functional configurations of a radiobase station according to the present embodiment. As shown in FIG. 17,as functional configuration pertaining to CSI feedback, a radio basestation 10 has a CSI-RS generating section 101 a (example 1.1, example1.2 and example 1.3), a CRS generating section 101 b (example 2.1 andexample 2.2), a CSI receiving section 102 (receiving section), avertical PMI selection section 103 (selection section), a 3D precoder104/3D channel forming section 104, and a scheduling section 105 (whichdetermines UEs and the 3D precoders to use).

The CSI-RS generating section 101 a pre-codes channel state measurementreference signals (CSI-RSs) using different precoding weights pervertical beam, and maps the pre-coded CSI-RSs to different CSI-RSresources per vertical beam.

To be more specific, the CSI-RS generating section 101 a configures KCSI processes in association with K (K≧1) vertical beams, and outputsCSI process information related to the K CSI processes that areconfigured, and K pre-coded CSI-RSs. The CSI process information and Kpre-coded CSI-RSs that are output from the CSI-RS generating section 101a are transmitted to the user terminal 20, through K CSI processes(processes), by a transmission section that is constituted with thetransmitting/receiving section 13, the amplifying section 12 and thetransmitting/receiving antenna 11 of FIG. 15.

Note that the K CSI processes each have corresponding CSI-RS resources,and the K pre-coded CST-RSs are mapped to CSI-RS resources thatcorrespond respectively to K CSI processes.

The CRS generating section 101 b pre-codes cell-specific referencesignals (CRSs) using different precoding weights per vertical beam, andmaps the pre-coded CSI-RSs to different subframes per vertical beam.

To be more specific, the CRS generating section 101 b outputs the Kpre-coded CRSs in association with K (K≧1) vertical beams. The Kpre-coded CRSs output from the CRS generating section 101 b aretransmitted to the user terminal 20, in K subframes, by the transmissionsection constituted with the transmitting/receiving section 13, theamplifying section 12, and the transmitting/receiving antenna 11 of FIG.15.

The CSI receiving section 102 receives channel state information(CSI_(H)'s) that is fed back from the user terminal 20. These CSI_(H)'sindicate the states of the horizontal domain channels (horizontalchannels) formed by the vertical beams. The CSI_(H)'s include precodingmatrix indicators (PMI_(H)'s) that identify the precoding weights of thehorizontal channels, rank indicators (RI_(H)'s) that identify the ranksof the horizontal channels, and channel quality indicators (CQI_(H)'s)that identify the channel quality of the horizontal channels.

To be more specific, the CSI receiving section 102 may receive theCSI_(H) ⁽¹⁾ . . . CSI_(H) ^((x)) (x=K) of K horizontal channels formedby K (K≧1) vertical beams (example 1.1 and example 2.1). Also, the CSIreceiving section 102 may receive the CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((x))(x=M) of M horizontal channels showing good channel states, selectedfrom the K CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) (example 1.2). Also, the CSIreceiving section 102 may receive CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((x)) (x≦K)that show channel states better than a predetermined threshold value,selected from the K CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) (example 1.3 andexample 2.2).

Based on the CSI_(H)'s input from the CSI receiving section 102, thevertical PMI selection section 103 selects the PMIs (vertical PMIs) forforming the vertical beams to use in downlink communication with theuser terminal 20. To be more specific, the vertical PMI selectionsection 103 selects the vertical PMIs that indicate these precodingweights. The vertical PM's (PMI_(v)'s) are selected by using, forexample, the CQI_(H) ⁽¹⁾ . . . CQI_(H) ^((x)) that are included in theCSI_(H) ⁽¹⁾ . . . CSI_(H) ^((x)), and a predetermined function (forexample, argmax). Note that the vertical PMI selection section 103constitutes a selection section that selects the PMIs (precodingweights) for forming one vertical beam or a plurality of vertical beamsto be used in downlink communication.

The 3D precoder/3D channel forming section 104 carries out precoding bymeans of vertical precoders, using the vertical PMIs selected in thevertical PMI selection section 103, and forms vertical beams. Also, the3D precoder/3D channel forming section 104 carries out precoding bymeans of horizontal precoders, and forms horizontal beams. The verticalbeams and the horizontal beams that are formed constitute 3D channels.Note that the 3D precoder/3D channel forming section 104 constitutes aforming section that forms 3D precoding weights using the PMIs(precoding weights) for forming horizontal beams, corresponding to thevertical PMIs selected by the vertical PMI selection section 103.

The scheduling section 105 carries out scheduling based on inputinformation from the 3D precoder/3D channel forming section 104.

FIG. 18 is a diagram to show the functional configurations of a userterminal according to the present embodiment. As shown in FIG. 18, asfunctional configurations pertaining to CSI feedback, a user terminal 20has a CSI-RS receiving section 201 a and a channel estimation section202 a (example 1.1, example 1.2 and example 1.3), a CRS receivingsection 201 b and a channel estimation section 202 b (example 2.1 andexample 2.2), a CSI generating section 203, a CSI feedback section 204(example 1.1 and example 2.1), and a CSI selection/CSI feedback section205 (example 1.2, example 1.3 and example 2.2).

The CSI-RS receiving section 201 a receives channel state measurementreference signals (CSI-RSs) that are pre-coded using different precodingweights per vertical beam. To be more specific, the CSI-RS receivingsection 201 a carries out the receiving processes of K pre-coded CSI-RSs(demodulation, decoding and so on) based on CSI process information thatrelates to K CSI processes in association with K (K≧1) vertical beams.

The channel estimation section 202 a performs channel estimation basedon the CST-RSs received in the CSI-RS receiving section 201 a. To bemore specific, based on the K pre-coded CSI-RSs input from the CSI-RSreceiving section 201 a, the channel estimation section 202 a estimatesK channels in the horizontal domain (horizontal channels), which areformed respectively by the K vertical beams (CSI processes).

The CRS receiving section 201 b receives cell-specific reference signals(CRSs) that are pre-coded using different precoding weights per verticalbeam. To be more specific, the CRS receiving section 201 b performs thereceiving processes (demodulation, decoding and so on) of K pre-codedCRSs that are transmitted in K subframes respectively.

The channel estimation section 202 b performs channel estimation basedon the CRSs received in the CRS receiving section 201 b. To be morespecific, based on the K pre-coded CRSs input from the CRS receivingsection 201 b, the channel estimation section 202 b estimates Khorizontal channels, formed by the K vertical beams respectively. As hasbeen described above, a horizontal channel is a 2D channel in thehorizontal domain that is formed by a vertical beam.

The CSI generating section 203 generates channel state information (CSI)of the horizontal channels estimated in the channel estimation sections202 a and 202 b. To be more specific, the CSI generating section 204generates the CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) of the K horizontalchannels that are formed by the K vertical beams respectively. Note thatthe CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) include the PMI_(H) ⁽¹⁾ . . .PMI_(H) ^((K)), the RI_(H) ⁽¹⁾ . . . RI_(H) ^((K)), and the CQI_(H)(1) .. . CQI_(H) ^((K)).

The CSI feedback section 204 performs the transmission processes (forexample, coding, modulation and so on) of all CSIs generated in the CSIgenerating section 203 (example 1.1 and example 2.1). To be morespecific, the CSI feedback section 204 carries out the transmissionprocesses of the CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) of the K horizontalchannels input from the CSI generating section 203 and outputs theresults. The CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) of K horizontal channelsthat are output are transmitted to the radio base station 10 by atransmission section constituted with the transmitting/receivingsections 23, the amplifying sections 22, the transmitting/receivingantennas 21 of FIG. 16.

The CSI selection/feedback section 205 selects CSIs that fulfillpredetermined conditions, from the CSIs generated in the CSI generatingsection 203, and carries out the transmission processes (for example,coding, modulation and so on) of the selected CSIs. To be more specific,the CSI selection/feedback section 205 may select M CSI_(H) ⁽¹⁾ . . .CSI_(H) ^((x)) (x=M) that show good channel states, from the CSI_(H) ⁽¹⁾. . . CSI_(H) ^((K)) of K horizontal channels input from the CSIgenerating section 203 (example 1.2). Also, the CSI selection/feedbacksection 205 may select CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) (x≦K) that showbetter channel states than a predetermined threshold value, from amongthe CSI_(H) ⁽¹⁾ . . . CSI_(H) ^((K)) of K horizontal channels input fromthe CSI generating section 203 (example 1.3 and example 2.2). TheCSI_(H) ⁽¹⁾ . . . CSI_(H) ^((x)) that are selected are transmitted tothe radio base station 10 by a transmission section that is constitutedwith the transmitting/receiving sections 23, the amplifying sections 22and the transmitting/receiving antennas 21 of FIG. 16.

The present invention is by no means limited to the above embodimentsand can be implemented in various modifications. For example, it ispossible to adequately change the number of carriers, the bandwidth ofthe carriers, the signaling method, the number of processing sections,the process steps and so on in the above description, without departingfrom the scope of the present invention. Besides, the present inventioncan be implemented with various changes, without departing from thescope of the present invention.

The disclosure of Japanese Patent Application No. 2012-197745, filed onSep. 7, 2012, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

1-12. (canceled)
 13. A user terminal comprising: a channel estimationsection that performs measurement on the basis of one or more resourcesof measurement reference signals that are each precoded with acorresponding precoding weight; and a generating section that generatesCSI on the basis of a result of the measurement.